1. Field of the Invention
The present invention relates to inertial navigation instruments. More particularly, this invention pertains to an accelerometer wherein the force experienced by a proofmass affects the modes of light within a non-planar optical cavity.
2. Description of the Prior Art
A large variety of accelerometers exists. While a wide range of shapes and sizes is possible, each works on the principle of the inertial "pushing back" of a known mass in response to an externally applied acceleration force. While this common principle relates to all accelerometers, mechanical, electromechanical and those formed of silicon semiconductor material, methods and apparatus for measuring the amount of acceleration experienced by the proofmass of the system vary considerably. For example, in force-rebalance systems wherein the proofmass is maintained in a null or near-null position, the amount of current required to actuate the system's torquers to maintain a null position provides an indication of the amount of acceleration force.
The accelerometer often forms a portion of an inertial navigation unit or system. In such a system, rate measuring gyroscopes may also be employed, often serving to stabilize the system's accelerometers. A significant advance in the gyroscope art has been the development of the ring laser gyroscope. In the ring laser gyroscope, two monochromatic laser beams rotate in opposite directions in a closed optical path. Angular movement of the gyroscope (and the body to which it is fixed or "strapped down") about its sensing axis results in an effective path length change for each beam, increasing the path for the beam travelling in the direction of the angular movement and decreasing the path of the beam travelling in the opposite direction by the same amount. As a result, the wavelengths of the two beams will change accordingly and this is reflected in changes in the beams' frequencies. The resulting frequency differential is proportional to angular rate.
The increase use of such laser technology in inertial navigation, coupled with the fact that such technology is physically quite different from existing accelerometer technologies, points to the potential future advantages that may accrue from the development of a laser accelerometer that is generally compatible with the laser gyro technology. Such advantages include common manufacturing technology, the potential fabrication of totally integrated laser inertial navigation systems on a single optical block, the inherently wideband operation of laser inertial sensors and the like. These potential benefits have in the past prompted attempts to develop a laser accelerometer. Generally, such attempts have featured an implementation of Hooke's Law wherein amorphous material is inserted into a resonant cavity and the stress-induced birefringence therein measured by observing the beat frequencies between two polarizations lasing in the cavity. Such work is described in U.S. Pat. No. 4,048,859 of Gary D. Babcock entitled "Laser Accelerometer."
The insertion of intracavity elements necessarily complicates the manufacture of precision optical instruments. More significantly, such inserted elements are lossy, resulting in increased energy consumption, lessening the efficiency and accuracy of the instrument.